The present invention relates to an adaptive coupling and an adaptive coupling method for joining a conduit to a barrier sheet through which the conduit passes and which is movable axially relative to the conduit. More particularly, the present invention relates to an adaptive coupling and method for use in gas recovery systems for landfill gas emissions wherein a substantially leakless seal is desired between a flexible barrier sheet covering a landfill and a gas recovery conduit extending through the barrier sheet into a landfill gas well or a monitoring conduit extending through the barrier sheet into a landfill monitoring well.
In a conventional landfill, the waste material is covered by a flexible barrier sheet or cap that is substantially gas impermeable. As the waste below the cap decomposes, landfill gases including methane and carbon dioxide are produced. The methane may create a risk of explosion if entrapped in an enclosed space under the landfill cap or if the gas migrates through subsurface formations to enclosed structures near the landfill.
The risk of explosion typically is mitigated by passive systems for venting and flaring the gas or by active gas recovery systems. Landfills with active gas recovery systems generally have monitoring wells and gas recovery wells strategically positioned near the perimeter of the landfill and also may have such wells throughout the landfill. A network of header pipes connect the wells to a pumping facility designed to remove the gas under vacuum from the landfill.
The typical gas recovery well has a vertical bore approximately two to three feet in diameter extending to a substantial depth in the landfill. Well depths ranging from approximately fifty percent to ninety percent of the depth of the landfill are typical and may be less than twenty-five feet in depth to more than one-hundred feet in depth. A gas recovery riser (or conduit) extends from the bottom of the well to the header pipes. Before reaching the header pipes, the gas recovery conduit typically passes through several feet of intermediate cover over the landfill waste material, a barrier sheet (or cap) over the immediate cover, and several feet of protective soil over the cap. Even though an active gas recovery system may be in place, a significant amount of landfill gas may be lost to the atmosphere. The loss may be attributable, in part, to leakage that occurs between the barrier sheet and the gas recovery conduit extending through the barrier sheet as the barrier sheet moves axially relative to the conduit due to the subsidence of the landfill material.
Accordingly, there is a need in the art for a coupling able to accommodate landfill subsidence while maintaining a substantially leakless seal between a barrier sheet and a gas recovery conduit.